1. Field of the Invention
The present invention relates to an optical information recording medium for recording/reproducing information by irradiation of laser light, and further relates to a method and an apparatus for recording/reproducing information on this medium.
2. Description of the Prior Art
Optical information media have been noted widely as a large capacity and high density memory, and erasable media that can be rewritten have been under development at present. In one of the erasable optical information recording media, a recording layer made of a phase change material that changes between an amorphous state and a crystalline state is formed on a substrate, and information is recorded and erased by irradiating the recording layer with laser light.
An alloy film comprising Ge, Sb, Te, or In as the main components such as a GeSbTe alloy is known as a phase change material for the recording layer. Information is recorded by making the recording layer partially amorphous to form recording marks. The recording layer is made amorphous by heating the recording layer to the melting point or more and cooling it. The recording layer is made crystalline by heating the recording layer to the crystallization temperature or more and not more than the melting point. The reflectance and the transmittance of the irradiated laser light in a region in which a recording mark is formed are different from those in other regions.
In general, spiral or concentric circular guide grooves that are tracked by laser light for recording/reproducing information previously are provided on a substrate. Regions between the grooves are called lands. In recordable CDs (CD-R) or minidisks (MD), either grooves or lands are used as information tracks to record information on, and the other is used as a guard band for separating adjacent information tracks.
In recent years, with improvement of the processing ability of various information equipment, the amount of information to be handled has increased. For this reason, there is a great demand for recording media with a larger capacity than ever. As one approach to achieve larger capacity, in DVD-RAMs or the like, a method for increasing track density by recording information on both grooves and lands is adopted (land & groove recording method).
In DVD-RAMs, a format structure called a ZCLV (Zoned Constant Liner Velocity) method is adopted. In this format structure, an information track is segmented into a large number of information recording regions (sectors) by addresses indicating the position on the medium. Further, several information recording regions are grouped in the radial direction to form a zone. The number of sectors corresponding to one rotation and the rotation speed are increased step by step by each zone from the inner circumference to the outer circumference. In each zone, the rotation speed is constant, and the linear velocity and the duration of a sector are substantially constant between the zones.
On the other hand, in CD-Rs or MDs, a CLV (Constant Liner Velocity) method is used in which information is recorded/reproduced at a constant linear velocity throughout the recording regions. This method can achieve the maximum recording density, and has an advantage in that the design of the recording layer is easy because the thermal conditions during recording are constant.
In the CLV method, it is necessary to change the rotation speed frequently for random access. For this change, the grooves are wobbled in the radial direction at a constant spatial frequency, and a rotating motor of a recording/reproducing apparatus is controlled based on signals obtained from these wobbles for recording/reproducing information. However, the length of a groove for one rotation is varied with the position in the radial direction, so that a phase difference occurs between the wobbles in the adjacent grooves. Therefore, when the land & groove recording method is used with the CLV method, signals are synthesized from wobbles having different phases in the land portion. Thus, good circuit control signals cannot be obtained.
In order to solve this problem, JP 6-338066 has proposed a recording medium in which only one edge of a groove is wobbled. In this recording medium, address information is recorded as a signal that has been frequency-modulated, using a rotation control signal as the carrier frequency.
However, when a gap between information tracks (track pitch) is narrowed to achieve higher density, the ratio of the variation of the information track width is increased, if the amplitude of the wobble is unchanged. Therefore, the signal amplitude is varied during reproduction of information, so that signal quality is deteriorated. On the other hand, when the amplitude of the wobble is reduced as well, the intensity of the signal obtained from the wobble is reduced, which makes it difficult to detect address information.
Furthermore, as another approach to achieve higher density information recording, multilayered recording media including at least two recording layers are proposed. In many multilayered recording media, it is necessary to record/reproduce information with laser light that has passed through another recording layer. Furthermore, it may be necessary to record information successively on a plurality of recording layers, or reproduce information continuously from a plurality of recording layers.